Active sites of H-ZSM5 catalysts for the oxidation of nitric oxide by oxygen

Abstract

The catalytic activities of H-ZSM5-18, H-ZSM5-150, Li-ZSM5-18, and H-Mag (numerical suffixes mark the Si/Al ratios of zeolites, H-Mag is the proton exchanged form of the layered sodium silicate, magadiite) were compared for the oxidation of NO by O2 at different ratios of reactants at reaction temperatures from 25 to 600°C. H-ZSM5-18 typically has activity maxima near 25 and 400°C at most O2/NO reactant ratios. Regardless of the partial pressures of reactants, NO±1/2O2 ⇋ NO2 equilibria are attained at 400°C and above. The H-ZSM5-150 and Li-ZSM5-18 zeolites are only active at temperatures near 25°C. H-Mag is practically inactive at the reaction conditions used. Results indicate that Lewis acidic lattice aluminium ions and silanol hydroxyls are not active in the oxidation of NO to NO2 over H-ZSM5 zeolites. Bronsted acidic bridging hydroxyls are probably active sites for this reaction at temperatures above 200°C.